The present invention relates to a method and arrangement for generating ignition sparks in an internal combustion engine provided with an ignition system in which there is included at least one spark plug obtaining ignition voltage from the secondary winding of an ignition coil, which ignition system comprises at least one ignition capacitor, which cooperates with at least one discharging circuit and one charging circuit, which discharging circuit comprises in series the primary winding of the ignition coil and a first circuit-breaking element switchable from a control unit, in which connection, at a first time, the control unit emits a first output signal to the first circuit-breaking element, which triggers the dis-charging of the ignition capacitor via the discharging circuit, in which connection there is produced, in the secondary winding, a first ignition voltage generation corresponding to this.
Such methods or arrangements are previously generally known and applied to capacitive ignition systems in internal combustion engines. The ignition spark which is obtained in a capacitive ignition system is relatively powerful and of short duration. Under certain operating conditions, such as idle running, it is difficult for such a spark to ignite reliably the fuel/air mixture in the engine cylinders, particularly if the mixture is relatively lean. For this reason, various solutions have previously been proposed for prolonging the duration of the ignition spark in a capacitive ignition system. Thus far, U.S. Pat. No. 3,906,919 describes a solution in which the primary winding of the ignition coil is divided into two separate windings where one winding is included in the discharging circuit of an ignition capacitor while the second winding obtains oscillating current from an oscillator. The duration of the ignition spark is thereby controlled by the time during which the oscillating current passes through the said separate part of the primary winding.
A similar solution with a divided primary winding is also described in U.S. Pat. No. 3,972,315. The ignition spark is produced by a combination of inductive and capacitive ignition voltage generation. In U.S. Pat. No. 4,258,296, use is made of a circuit-breaker between the primary winding of an ignition coil and an ignition capacitor in order to permit simultaneous ignition voltage generation in the ignition coil both inductively and capacitively.
However, all the said known solutions comprise a large number of components, inter alia for creating an oscillating current and for charging the capacitor and current supply via the primary winding. Furthermore, the solutions are suited for relatively large ignition coils primarily intended for inductive ignition and wherein a single ignition coil is used for all the spark plugs of the engine, at the same time as the current to these is distributed by means of a conventional ignition distributor.
The present invention is primarily applicable to a microprocessor-controlled capacitive ignition system for multiple cylinder engines for vehicle operation. In addition, the invention is used with advantage in an ignition system of this type which does not have mechanical ignition distribution and in which an ignition coil of relatively small size is used for each spark plug. For the purpose of prolonging the duration of the ignition spark in a simple manner in ignition systems of this type, the present invention is characterized in that, at a second time which occurs later than the first time, the control unit emits a second output signal to a second circuit-breaking element located in the charging circuit, which element is connected in series to the discharging circuit, by which means the first and second circuit-breaking elements are simultaneously kept conductive for current supply from an electrical energy source via the primary winding and the said circuit-breaking elements, and in that, at a third time, which occurs later than the second, the control unit emits a third output signal to the first and/or second circuit-breaking element for a nonconductive state of this or these, in which connection the current supply via the primary winding is interrupted and a second ignition voltage generation takes place in the secondary winding.
By means of the invention, a microprocessor-based control unit can be simply programmed for controlling the two circuit-breaking elements in such a way that the method according to the invention is achieved. It is also possible to allow the microprocessor to adapt the control of the circuit-breaking elements to the operating condition of the engine, by which means the duration of the ignition spark can be changed in dependence on changes in the operating condition.
In a preferred arrangement of the invention as described above, the capacitor is thereby electrically connected to, on the one hand, at least one discharging circuit comprising the primary winding of the ignition coil connected in series to a first circuit-breaking element and, on the other hand, a charging circuit which comprises a second circuit-breaking element, a coil and at least one diode.
The arrangement according to the invention is characterized in that the first and second circuit-breaking elements and the primary winding are connected in series to each other in a circuit which connects a direct-current source to earth, via which circuit direct current flows when both the first and the second circuit-breaking elements are conductive for passage of current, and in that the ignition capacitor is electrically connected to the first circuit-breaking element and the primary winding so that, when the first circuit-breaking element is conducting, the ignition capacitor discharges via the primary winding. The circuit-breaking elements are electrically connected to an electronic control unit which, in dependence on input signals representing the operating condition of the engine, emits output signals to the circuit-breaking elements to achieve a conductive or nonconductive state of the same.
The arrangement according to the invention provides a particularly simple and inexpensive solution for producing a prolonged ignition spark.